What causes the signet ring sign on HRCT of the chest?
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Answer:
The signet ring sign is a dilated bronchus seen in cross-section lying adjacent to its accompanying pulmonary artery branch, with the bronchus larger than the artery, so that the pair resembles a signet ring: the ring is the bronchial lumen and the jewel is the smaller vessel. It is the cardinal HRCT criterion for bronchiectasis. It is quantified as the bronchoarterial ratio, the internal bronchial diameter divided by the diameter of the adjacent artery. A ratio above 1.0 is widely quoted, but no universally validated cut-off exists: values between 1.0 and 1.5 occur in healthy people, and a ratio above 1.5 is much more convincing. The ratio is also raised without disease by increasing age, residence at high altitude and chronic asthma, and it can be spuriously elevated whenever the artery is small, as in hypoxic vasoconstriction or reduced regional perfusion. It should therefore be read together with the other criteria: lack of normal peripheral tapering and visibility of bronchi within 1 cm of the pleura.
Why is it called so?
The circular dilated bronchus with the smaller round artery abutting it looks like a signet ring with its raised jewel.
Pathophysiology
Chronic infection and inflammation destroy the elastic and muscular components of the bronchial wall and the surrounding peribronchial tissue, so the airway loses its ability to resist the traction of the surrounding lung and dilates permanently. The accompanying pulmonary artery branch, whose calibre is set by the flow it carries rather than by airway inflammation, keeps roughly its normal size, and may even be reduced by hypoxic vasoconstriction in the poorly ventilated segment. The reversal of the normal relationship, in which the two are approximately equal, produces the disproportion the sign records.
Alternative names: Signet ring appearance, bronchoarterial ratio sign
Other associated named signs: Tram track sign, string of pearls sign, lack of tapering, tree-in-bud, and the mucoid impaction or gloved finger sign
References
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
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